1. Field of the Invention
The present invention is directed to a 3D angio-volume reconstruction method for a three-dimensional subject based on 2D projection exposures.
2. Description of the Prior Art
Shadow artifacts that are equivalent to metal artifacts occur in native 3D angio-volume reconstruction (volume reconstruction based on 2D projection exposures that are acquired with a C-arm apparatus after the injection of the contrast agent). Problems associated therewith are a very low signal-to-noise ratio in the shadow region, a greatly increased relationship of stray radiation to primary radiation, and beam hardening artifacts. These effects lead to dark locations in the image between the vessels filled with contrast agent and lead to stripes that extend over the entire reconstructed region. In particular, the image clarity in the immediate environment of the vessels filled with contrast agent is greatly reduced.
Metal artifacts have long been a problem in conventional CT methods; there are various known correction approaches for these artifacts. These correction methods have in common the fact that metal is viewed as an opaque object due to the finite detector dynamics. Accordingly, the attenuation values measured in the metal shadows are completely ignored and considered unusable. The xe2x80x9cholexe2x80x9d projections that thus arise must, however, then be supplemented in order to obtain a complete dataset for the back-projection. The most widespread algorithm from 2D-CT (MAR algorithm) bridges the missing one-dimensional projection data by means of a linear function. Such a procedure, however, is far too complicated in an angio-volume reconstruction, the individual projections of which are acquired with a DSA apparatus (digital subtraction angiography), because of the 2D character of the projection images.
An object of the present invention is to provide a volume reconstruction method wherein shadow artifacts can be eliminated in a simple way.
This object is inventively achieved by producing a 3D angio-volume dataset V1 based on 2D mask images, and a 3D angio-volume dataset V2 based on 2D fill images, and the vessel tree is isolated in the fill volume dataset V2 by means of a segmentation and is added to the mask image volume dataset V1 after scaling.
According to the inventive method, thus, both mask images, i.e. projection exposures without contrast image, as well as fill images, i.e. projection exposures with contrast agent, are generated in a standard DSA exposure sequence. Due to the absence of contrast agent in the vessels when the mask images are obtained, shadowings do not occur in the volume dataset V1 based on mask images. As a result of superimposition of the segmented vessel tree from the fill volume dataset V2, thus, a complete 3D angio-volume dataset can be produced without any shadowing artifacts whatsoever. A particular advantage of the inventive reconstruction method is that the combination of the 3D datasets can ensue by simple addition. A registration step is not needed for this purpose since both V1 as well as V2 are reconstructed with reference to the same coordinate system. An extremely simple method thus is achieved from presenting low-contrast or medium-contrast subjects artifact-free together with high-contrast vessel trees.
Segmenting of the vessel tree can ensue directly from the fill volume dataset V2, for example by simple thresholding, i.e. setting of thresholds, or by means of region growing, i.e. combining neighboring and/or related voxels (volume elements), with the result being subsequently refined by morphological operations. It has proven especially expedient, however, in a further embodiment of the invention to produce the segmenting of the vessel tree from the difference volume dataset V=V2xe2x88x92V1.